IDAT

IDAT is a chunk type in the Portable Network Graphics (PNG) image format that carries the actual image data. The data contained in one or more IDAT chunks is a zlib-compressed deflate stream. All IDAT chunks for a single image are read in file order and concatenated to form the complete compressed data stream, which is then decompressed to yield the image pixel data.

When decompressed, the image data is organized as a sequence of scanlines. Each scanline begins with a filter type byte (values 0 through 4) that indicates how that line was preprocessed during compression. The filters—None (0), Sub (1), Up (2), Average (3), and Paeth (4)—are used to improve compression by exploiting redundancy between adjacent pixels. After reversing these filters, the raw pixel values are obtained according to the image’s color type and bit depth, as described in the IHDR chunk.

IDAT chunks are part of the PNG file’s chunk-based structure. A PNG file typically includes an IHDR chunk (image header), optional PLTE (palette) for indexed color, one or more IDAT chunks with the image data, and an IEND chunk marking the end of the file. Each IDAT chunk has a 4-byte length, the 4-byte type code “IDAT,” the data payload, and a 4-byte CRC covering the type and data. The presence of multiple IDAT chunks is common, and they must be processed in sequence to reconstruct the full image data. Decoding thus involves concatenating all IDAT data streams, decompressing, and reversing the per-scanline filters to recover the final image pixels.